🛡️ Sentinel: Structural validation for save data

[b0x1]

This PR enhances the security of the save system by implementing structural validation for deserialized game state.

🚨 Severity: MEDIUM
💡 Vulnerability: Insecure deserialization of save game data from localStorage using blind type casting.
🎯 Impact: Malformed or maliciously modified save data could cause the application to crash or enter an inconsistent state when loaded.
🔧 Fix: Added a isValidSaveData type guard that verifies the presence and types of essential SaveData properties (players, map, turn, phase, etc.) after JSON parsing but before the data is used.
✅ Verification:

• Manual verification of src/game/systems/SaveSystem.ts structure.
• Ran all project tests via pnpm test, confirming SaveManager.test.ts and SaveSystem.test.ts pass.
• Verified that invalid/partial JSON objects now return null instead of being cast to SaveData.

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PR created automatically by Jules for task 5394744797254544891 started by @b0x1

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